1. Field of the Invention
The present invention generally relates to optical limiter enhancements and more specifically, to performance enhancement systems and techniques fox photorefractive crystal optical limiters.
2. Description of Prior Art
Photorefractive crystals as power limiters continue to have performance problems with regards to time response and beam depletion. The time response for photorefractive beam fanning depends inversely on the incident intensity which determines the limiting threshold. When the time response is optimized there is a proportional decrease in the limiting threshold. If ample time is allowed for the limiting to occur even very weak cw (continuous wave) beams (approx. 1 milliwatt) can be limited. The particular photorefractive process of asymmetric self-defocusing or "beam fanning" which results from wave mixing or beam coupling has been exploited as a sensor protection device. The photorefractive process arises from the: second-order nonlinear optical susceptibility and has a distinguishing feature, in that the time response depends on the intensity. Improvement in the time response depends on the incident intensity (I) according to the relationship: EQU .tau.=A/I.sup.x
where:
x is approximately unity, and
A is a material parameter with units of energy density.
Since x is approximately unity, the material parameter A determines the limiting threshold. An example of photorefractive crystal power limiters may be found in U.S. Pat. No. 5,073,705 to Sharp et al. entitled "Broadband, Multi-line, Optical Power Limiting Scheme" issued 17 Dec. 1991, incorporated herein by reference.
While the prior art has reported using optical limiters based on photorefractive limiters none have established a basis for a specific apparatus that is dedicated to the task of resolving the particular problem at hand. What is needed in this instance is a beam fanning limiter system and technique which achieves improved time response and maximum interaction length. This would yield truly passive broadband sensor protection against high intensity, short-pulse, high repetition rate multi-line lasers and multi-line cw lasers.